Rate responsive cardiac pacemakers which measure a physiologic parameter indicative of metabolic demand and vary the pacing rate as a function thereof are well-known. The relationship between a parameter such as oxygen saturation of the blood and a patient's corresponding metabolic demand and desired heart rate is also well-known. Consequently, pacemakers which regulate pacing rate in response to sensed blood oxygen level and attempt to restore the desired relationship between blood oxygen level and pulse rate in order to meet a patient's physiological demands are believed particularly desirable for individuals requiring rate responsive pacemaker support.
A thorough description of demand pacemakers having such oxygen-sensing capabilities utilizing dual-wavelength, reflectance oximetry technology can be found, for example, in U.S. Pat. No. 4,903,701, issued to Moore et al., as well as in U.S. Pat. No. 4,750,495, issued to Moore et al.
While the performance of such prior devices has been generally acceptable, some concern exists that there may be undesired susceptibility to noise riding upon the signal being monitored. The problem presents itself when attempting to balance the need for maintaining adequate signal sensitivity (i.e., by setting the comparator threshold of the sensor signal comparator sufficiently "close to" the expected signal amplitude levels expected to be sensed to promptly detect appropriate signal level transitions) with the need of maintaining adequate noise immunity (i.e., by setting the comparator threshold of the sensor signal comparator sufficiently "far away from" the expected signal amplitude levels expected, together with noise riding thereon, to avoid inappropriate detection by the comparator).
Another problem is the extent to which the sensitivity of such sensor-based devices may vary from unit to unit, unless each unit's comparator threshold has been uniquely established at an amplitude which is appropriate for the particular sensor signal amplitudes generated by that particular unit's sensor circuitry.
A further problem can develop, for example, with respect to the same unit over time due to aging effects upon critical optical components required to produce light emissions and sense reflected light, such that the signal amplitudes generated by such unit may drift to an unacceptable extent with respect to what is typically a relatively fixed-level comparator threshold.
Therefore, what is needed is a monitoring system for such devices, particularly such as for the subject oxygen-sensing, dual-wavelength, reflectance oximetry based, rate responsive cardiac pacemaker disclosed herein, wherein continual and automatic adjustments are made by the monitoring system to the sensor signal comparator threshold, such that the effects of noise, sensitivity and drift on oximeter output signals sent to the pacemaker are minimized.